The present invention relates to apparatus for registering and feeding sheet material, e.g., sheets of paper, to a work station, e.g., the print station of a document printer. More particularly, it relates to a sheet registering and feeding apparatus which is particularly useful in a duplex document printer for re-registering and feeding image-receiving sheets to a work station upon having one side thereof previously presented.
Duplex document printers and copiers are well known in the art. See, for example, the printers and copiers disclosed in U.S. Pat. Nos. 4,453,840; 4,348,101; 4,305,655; 4,330,197; and 4,140,387. Virtually all such printers and copiers include sheet-feeding apparatus adapted to sequentially present both faces of a sheet to a work station (e.g., the print station of a printer, or the exposure and transfer stations of an electrophotographic copier).
The presentation of both faces of a sheet to a work station typically involves the steps of (a) inverting the sheet between successive presentations so that, after presenting one face to the work station, the opposite face is in a position for presentation, and (b) reversing the sheet's lead/trail edge orientation, so that both faces of the sheet are advanced in the same direction past the work station. Usually added to these steps is a third step of re-registering the sheet (e.g., to eliminate skew or to time its advance with some other machine activity) prior to being presented to the work station for a second time.
Apparatus for reversing a sheet's lead/trail edge orientation typically comprises means for feeding a sheet into and out of an arcuate guideway or chute having a closed end. Upon advancing the sheet out of the chute in a direction opposite that which it entered the chute, the sheet's trailing edge (as it entered the chute) now becomes the leading edge, and vice versa. The apparatus disclosed in U.S. Pat. No. 4,262,895 is typical of those devices used for reversing a sheet's lead/trail edge orientation. Such apparatus includes a three roller system which functions to advance sheets into and out of the edge-reversing chute. The three rollers are arranged in a stack and cooperate to define input and output nips at the chute entrance. The central roller of the stack is driven in a direction opposite that of its adjacent rollers. Thus, the central roller cooperates with one of its adjacent rollers to feed sheets into the chute, and cooperates with the other adjacent roller to feed sheets out of the chute. Since the distance between the roller's input nip and the chute's closed end is less than the distance between the sheet's lead and trail edge, a sheet fed into the chute will begin to buckle within the chute before the then trailing edge clears the input nip. Ideally, as soon as the trailing edge of the sheet clears the input nip, the restoring forces in the sheet cause such edge to move abruptly to the output nip, whereby the sheet is advanced out of the chute.
While apparatus of the above type functions well in reversing a sheet's lead/trail edge orientation, it will, under certain conditions, introduce an undesired skew into the sheet as it exits from the reversing chute. For example, if the closed end of the chute is not perpendicularly aligned with the feed path, the sheet will tend to skew as it buckles within the chute. Thus, upon becoming free of the roller's input nip, the sheet's trailing edge will snap into the output nip with one side of the edge arriving at the output nip before the other. The result, of course, is that the sheet will be fed from the chute, one side before the other, i.e., in a skewed manner. Also, even when the chute's closed end is perfectly aligned relative to the sheet path, there is a chance that skewing of the sheet will still occur since the roller system actually loses control over the sheet between the time the sheet's trailing edge clears one nip and enters the other. If, for any one of a variety of reasons, one side of the trailing edge of the sheet arrives at the output nip before the other, sheet skew will be introduced.
In the commonly-assigned U.S. Pat. No. 4,223,886, a similar apparatus is disclosed for reversing a sheet's lead/trail edge orientation. In this case, a two-roller system is employed for advancing a sheet into and out of a reversing chute. The rollers are selectively driven in opposite directions to advance sheets into and out of the chute. To effect registration of the sheet as it exits the chute, a registration gate is moveable into and out of the sheet path downstream of the roller pair. In this case, the roller pair advances the sheet into the chute only until the trailing edge of the sheet passes the registration position. At this point, the registration gate is moved into the sheet path and the roller pair is driven in the opposite direction to advance the now leading edge of the sheet into contact with the gate. While control over the sheet is maintained by the roller pair at all times, accurate registration of the sheet against the registration gate relies upon a slipping, frictional engagement between the rollers and the sheet. Since sheet's of different texture give rise to different frictional forces, no single pressure setting can be made which will assure a reliably registered feeding of all type of sheets. In this particular patent, any skew which may be introduced in the sheet as it is fed from the reversing chute is removed further downstream by a registration gate positioned at the work station.